Parallel platform structures have found use in such applications as flight simulators, manipulators and hand controllers. These structures may typically be envisaged as having two triangular platforms: a top platform which is free to move in relation to a base platform which is fixed. These platforms are typically connected to each other at their corners via six linear actuators to form an octahedral structure. By altering the length of the linear actuators it is possible to locate the top platform in a variety of positions or orientations with respect to the base platform. Parallel platform structures of this type have a top platform which has six degrees of freedom of movement. However it is difficult to design parallel platform structures which have large working volumes using linear actuators, and it is also difficult to incorporate adequate back-drivability into these actuators. These problems are particularily acute when designing compact parallel platform structures. Pantograph actuators which are operated by planetary geared motors have been proposed by H. Inoue et al as an alternative to linear actuators for use in parallel manipulators.
These pantograph actuators are of limited suitability for use in hand controllers due to their high torque noise which is a consequence of the use of a planetary gear mechanism. Also the varying inertia associated with different motor positions around the sun gear would be difficult to compensate for in a hand controller and would reduce its sensitivity. Also as the whole motor assembly is involved in the motion of any leg pair the effective inertia of the system is increased and sensitivity is again reduced. The inertia problems associated with planetary geared pantographic actuators make it difficult to provide the desired level of force feedback to devices incorporating these actuators. It is desirable that hand controllers are convenient to use and have adequate force feedback and back-drivability so as to minimize operator fatigue.